1. Field of the Invention
This invention relates to a method of providing a strong oxidizing solution containing sulfuric acid and peroxydisulfuric acid or H.sub.2 S.sub.2 O.sub.8 (PDSA) having relatively long shelf life and to the stored strong oxidizing solution.
2. Brief Description of the Prior Art
Caro's acid (H.sub.2 SO.sub.5) is a powerful oxidizing agent used in preparation of standard piranha etch solutions for use in semiconductor fabrication technologies. The piranha etch solution (Caro's acid) is generally used to remove photoresist from semiconductor devices being fabricated. Pirhana etch solution generally contains from about 4 to about 5 parts by volume concentrated H.sub.2 SO.sub.4 to one part by volume hydrogen peroxide. Caro's acid can be formed by mixing sulfuric acid (H.sub.2 SO.sub.4) and hydrogen peroxide (H.sub.2 O.sub.2) in a reversible reaction which yields the Caro's acid and water. In the required ratios, this reaction is exothermic, raising the temperature of the bath from ambient to about 110.degree. to about 130.degree. C. Caro's acid formulated in this manner is also relatively unstable and has therefore presented storage problems. In general, when Caro's acid has been required, it has been formulated and used immediately after formulation.
Oxidizing solutions have also been produced in the prior art by mixing peroxydisulfuric acid or H.sub.2 S.sub.2 O.sub.8 (PDSA) with sulfuric acid. This formulation has also been found to have poor shelf life because, after about 1 week, there was a 50 percent loss of product where a solution of 10 parts by volume concentrated sulfuric acid to one part by volume PDSA was used. Accordingly, this formulation has only been prepared and applied at the time and point of use.
The economics of piranha solution formulation would be materially improved if the strong oxidizing solution required could be an off-the-shelf item with materially extended shelf life by storage at the point of use rather than having to be formulated separately in conjunction with each use. Not only is the cost of preparation of strong oxidizing solution high but the cost of delivery in a pipeline is also quite high.